There are known a glow discharge process and a thermal induced chemical vapor deposition process as a method for the formation of a deposited film using a silicon hydride such as SiH.sub.4 and Si.sub.2 H.sub.6 as the starting material. These processes act a deposited film composed of A-Si is formed on a substrate by exciting and decomposing, or polymerizing such silicon hydride by the action of an electrical or thermal energy.
The thus formed deposited films are usable for various purposes.
However, in the case of forming a deposited film using the glow discharging process, it sometimes becomes difficult to establish the repeatable and stable film forming conditions because the resulting film will be influenced by a discharge energy during the film forming process particularly under a high power condition.
This problem will often occur in the case of forming a deposited film of large area or of greater thickness.
In the case of forming a deposited film by means of the thermal induced chemical vapor deposition process, limited kinds of substrates may be used because the deposition space must be maintained at a high temperature. In addition to this problem, it is difficult to obtain such a deposited film having the desired characteristics because the probability of bound, non-eliminated hydrogen atoms remaining in the resulting film composed of A-Si will decrease because of high temperature conditions.
In view of the above, there are still unsolved problems in the case of forming a deposited film by means of the glow discharge deposition process or the thermal induced chemical vapor deposition process, so that it is difficult to ensure uniform electrical and optical characteristics and the uniform quality of the resulting film. In addition to this, there are also other unsolved problems, such as in disorder or defects that occur at the surface of the resulting film or within the resulting film.
In order to solve the foregoing problems, there has been proposed a photochemical vapor deposition method using light energy (hereinafter referred to as "photo-CVD method") for the formation of a deposited film composed of A-Si.
In accordance with the photo-CVD method, there are advantages that a deposited film composed of A-Si can be formed through an ion-free chemical reaction at low temperature, and because of this, the foregoing problems can be greatly diminished.
Along with the photo-CVD method, there have been proposed various apparatuses for practicing said method. One such apparatus is of the kind that comprises a reaction chamber, raw material gas introducing means, means to supply light energy to a raw material gas in the deposition space of the reaction chamber and means to hold a substrate on which a deposited film is to be formed. A structure according to this method is shown in FIG. 4. Referring to FIG. 4, there are shown a luminous flux 1 of light energy supplied from a light source, light transmissive window 2, a deposited film 3, a substrate 4, deposition chamber 5, exhaust pipe 6 and raw material gas feed pipe 7.
However, for the foregoing apparatus, it is a problem that a A-Si material, which constitutes the resulting film on the substrate, is also deposited on the inner surface of the light transmissive window as foreign matter and this results in greatly reducing the strength of the incident light, thereby reducing deposition rate of a film to be formed on the substrate is reduced.
In order to solve this problem, it has been proposed that the inner face of the light transmissive window be coated with a vacuum pump oil. However, there are problems with or this proposed method, in that organic materials, including oily materials, often become dispersed in the deposition space of the deposition chamber during the film forming process. Such molecules are incorporated in a film to be formed. The resulting film is defective in quality and characteristics.
In order to solve the above problem, another proposal has been made that the inner face of the light transmissive window be periodically etched to thereby remove the foreign matter deposited thereon. Even for this method, there exist the problems that use of a raw material gas for the etching other than a film forming raw material gas is required because of this, the apparatus necessitates provision of an extra gas excitation apparatus to cause an etching reaction, such as an RF glow discharging apparatus. In view of this, the total scale of the apparatus in this case eventually becomes complicated, since the photo-CVD apparatus and the etching apparatus are combined.